Optical fiber for telecommunications comprise an outer part (cladding) of vitreous silica and an inner part (core) of silica doped with metal oxides having higher refractive index. Doping is generally performed by deposition techniques, such as CVD (Chemical Vapor Deposition).
Among the dopants, used are metal oxides which considerably raise the refractive index even in limited concentration and which, unlike germania, generally used for such fibers, do not give rise to the central refractive-index depression (dip) typical of fiber fabricated by deposition techniques. An example of such dopants is alumina.
Other dopants, such as rare earths, which modify the fiber emission or absorption characteristics so as to permit the fibers to be used as sensors, amplifiers or lasers in particular spectral regions, in particular in the infrared region, are often desiderably added to the dopants increasing the core refractive index.
The use of deposition techniques for manufacturing optical fibres of this kind gives rise to problems in the choice of reactants yielding the dopants, chiefly when alumina is dealt with, since no inorganic salts of aluminium exist which are liquid at ambient temperature and are easy to handle like Sicl4 or GeCl4. It is then necessary to use compounds with a melting point higher than 100.degree. C. and heated lines which presents a certain degree of complexity degree. As an alternative, organometallic salts of Al exist which are gaseous or liquid under normal conditions, but the chemical stability thereof is rather reduced. The same problem of unavailability of compounds with the desired characteristics is encountered when dealing with rare earths.
To solve these problems, it has been proposed to dope silica by using solutions of compounds of the dopant precursors. This technique has been described by J. E. Townsend, S. B. Poole and D. N. Payne in the article entitled "Solution-doping technique for fabrication of rare-earth doped optical fibres", Electronics Letters, Mar. 26th, 1987, Vol. 23, No. 7, pages 329-331.
According to the technique described in the cited paper, a number of layers of vitreous silica and other oxides are deposited in soot form inside a support tube, in an ambient isolated from the outside. The tube is subsequently removed from the lathe and the unvitrified layers are impregnated with aqueous solutions of compounds (in particular halides) of the rare earths or other required metals. The tube with the impregnated silica is rinsed with acetone to remove excess water, replaced in the lathe and submitted to a hightemperature treatment with Cl2 Core vitrification and preform collapsing are then effected in conventional manner.
This technique gives rise to a number of problems due to the use of aqueous solutions and to the tube removal from the lathe.
The use of aqueous solutions renders the vitreous matrix highly polluted with OH-groups, which are not completely removed by the elimination of excess water and dehydration with Cl2. Due to the presence of the OH-groups, the fibers have high attenuation. Besides, aqueous solutions are also unsuitable when the doping metal hydrolyzes yielding corresponding oxyacids or hydroxides. This is the case with aluminum, which is a very suitable dopant for the core of silica optical fibers for telecommunications, but whose compounds generally used as dopant precursors (e.g. AlCl.sub.3) violently react in H.sub.2 O.
Tube removal from the lathe causes contamination of the deposited material due to contact with the atmosphere of the production environment, and this reduces or even annuls the advantages of deposition in a closed environment. Moreover, the removal of the tube from and its subsequent replacement on the lathe are operations of a certain complexity, since they require cutting and resoldering of the glass tube; furthermore, these operations create further pollutions and are quite time-consuming, thus increasing manufacturing costs.